Halonitroaromatic compounds include various mono- and di-halo substituted nitroaromatics. Specific examples include chloronitroaromatic compounds such as 2-, 3-, and 4-chloronitrobenzene; 2,4-dicloronitrobenzene; 2,5-dicloronitrobenzene; 3,4-dichloronitrobenzene, among others. The catalytic hydrogenation of halonitroaromatics is a reaction of significant industrial importance because the resulting haloaminoaromatic compounds (e.g., haloanilines) are useful as intermediates in the production of certain agrochemicals, pharmaceuticals, and polymers. For example, 2,5-dichloroaniline can be used as an intermediate in the production of 3,6-dichloro-2-methoxybenzoic acid (also known by its common name dicamba), which is a highly effective and commercially important herbicide that is useful for controlling a wide variety of unwanted vegetation, including agricultural weeds. Convenient and economical methods of preparing dicamba, therefore, are of significant commercial importance.
Various processes for the catalytic hydrogenation of halonitroaromatics are described in references such as U.S. Pat. Nos. 3,073,865; 3,145,231; 3,291,832; 4,020,107; 4,760,187; as well as Kosak, “Hydrogenation of Haloaromatic Nitro Compounds,” Catalysis in Organic Synthesis, Academic Press, London, 1980, 107-117. One problem typically encountered during the hydrogenation process is the loss of selectivity to the desired haloaminoaromatic product through dehalogenation of the haloaminoaromatic product. Attempts to solve this problem have focused on introducing catalyst modifiers or additives into the reaction medium to suppress the dehalogenation reaction. For example, U.S. Pat. No. 3,073,865 describes the addition of a hydroxide or oxide of magnesium to the reaction medium as a dehalogenation suppressor. U.S. Pat. No. 3,145,231 discloses the use of cycloaliphatic amines such piperazine and morpholine as dehalogenation suppressors. U.S. Pat. No. 4,020,107 introduces an acidic phosphorous compound to suppress the dehalogenation reaction. U.S. Pat. No. 4,760,187 uses a ruthenium-platinum catalyst for halogenating chloronitrobenzenes to chloroanilines that also reduces dehalogenation. Also, Kosak teaches that the use of a sulfided platinum catalyst is one of the more effective procedures for minimizing dehalogenation.
Although these processes may be effective strategies for minimizing dehalogenation of the halonitroaromatic compound, these processes introduce additional components to the reaction medium or catalyst material. In multi-step processes that include the step of hydrogenating a halonitroaromatic, introducing additional components to the reaction mixture can affect downstream processes and reactions and may necessitate further separation operations, which increase process costs. Also, introducing catalyst modifiers can increase process costs and result in the presence of undesired metal contaminants or the production of other undesired reaction products.
Accordingly, there remains a need for processes for the catalytic hydrogenation of halonitroaromatic compounds that minimize the introduction of extraneous additives and catalyst modifiers but still provide for high selectivity to the haloaminoaromatic product and limit selectivity loss through dehalogenation.